Heterogeneous networks (HetNet) have been added to the scope of the LTE-A work item and now enhanced inter-cell interference coordination (eICIC) for co-channel HetNet deployment is one of the key technical points for Release 10.
Co-channel HetNets comprise macrocells and small cells operating on the same frequency channel. Such deployments present some specific interference scenarios for which new ICIC techniques are required.
In one scenario, the small cells are picocells, which are open to users of the macrocellular network. In order to ensure that such picocells carry a useful share of the total traffic load, user equipments (UEs) may be programmed to associate preferentially with the picocells rather than the macrocells, for example by biasing the SINR threshold at which they will select a picocell to associate with. Under such conditions, UEs near the edge of a picocell's coverage area will suffer strong interference from one or more macrocells. In order to alleviate such interference, some subframes may be configured as “blank” or “almost blank” in a macrocell. A blank subframe contains no transmission from the macrocell, while an “almost blank” subframe typically contains no data transmission and little or no control signaling transmission, but will contain reference signal transmissions in order to ensure backward compatibility with Release 8/9 UE which expects to find the reference signals for measurements but are unaware of the configuration of almost blank subframes. Almost blank subframes may also contain synchronization signals, broadcast control information and/or paging signals. For the simplicity, hereafter both blank and almost blank subframes are called as almost blank subframes (the term “ABSs” for short).
In order to make the use of ABSs effectively, signaling is needed to send the pattern of ABSs from the macrocell to the picocell across the corresponding backhaul interface, known in LTE as the “X2” interface. For LTE Release 10, it has been agreed that this signaling will take the form of a coordination bitmap to indicate the pattern of ABSs, for example with each bit corresponding to one subframe in a series of subframes, with the value of the bit indicating whether the subframe is an ABS or not. Please refer to R1-105779 “Way Forward on time-domain extension of Rel 8/9 backhaul-based ICIC” for details. Such signaling can help the picocell to schedule data transmissions in the picocell appropriately to avoid interference, for example by scheduling transmissions to UEs near the edge of the picocell during ABSs, and to signal to the UEs the subframes which should have low macrocellular interference and should therefore be used for RRM/RLM/CSI measurements.
The number of ABSs that need to be configured will depend on factors such as the traffic loading in the picocell. However, so far, no mechanism has been agreed to assist the macrocell to determine a useful number and pattern of ABSs to configure. In another similar scenario, the small cells are femtocells, which operate on a Closed Subscriber Group (CSG) basis, and are therefore typically not open to users of the macrocellular network. In this case, the femtocells can cause strong interference to the macrocell UEs when they come close to the femto eNodeBs. It may then be beneficial for the macrocells to request certain numbers of ABSs to be provided by the femtocells. In order to alleviate such interference, some subframes may be configured as ABSs.
Therefore, in this scenario, likewise, a mechanism is also required to assist the femtocell to determine a useful number and pattern of ABSs to configure.